ROCSat-2 is an NSPO (National Space Program Office) of Taiwan Earth imaging satellite with the objective to collect high-resolution panchromatic (2 m) and multispectral (8 m) imagery for a great variety of applications such as in land use, agriculture and forestry, environmental monitoring, natural disaster evaluation, and in support of research interests, in particular with the ISUAL instrument. Daily image coverage of Taiwan and the surrounding region is required.

Background: A contract was signed in May 1999 between NSPO and DASA/DSS (Dornier Satelliten Systeme GmbH) of Germany to build a high-resolution optical imaging satellite. However, the German government refused to give DASA/DSS an export licence for the S/C (the People's Republic of China was protesting the deal). The stalemate was resolved in Dec. 1999 when NSPO signed a new contract with MMS (now Astrium SAS of France). Quick approval of the export of ROCSat-2 was provided by the French government. The ROCSat program is part of a long-term effort in Taiwan to develop an autonomous space capability.

Note1: A public naming competition took place in Taiwan in 2004 with regard to the ROCSat satellite program. At the end of this contest, the ROCSat program was given the new name of FormoSat in December 2004. Hence, ROCSat-2 became FormoSat-2.

Note 2: In 2005, the National Space Program Office was renamed to "National Space Organization", keeping the same acronym, NSPO.

The spacecraft bus has been built by EADS Astrium SAS (prime contractor) of Vélizy, France, based on the Leostar 500 XO family. There were also contributions from Taiwanese industry (including satellite computers, S-band antennas, and sun sensors). The S/C structure consists basically of a hexagonal body of 1.6 m side length (diameter), 2.4 m in height.

The satellite is three-axis stabilized. The upper deck of the platform carries the payload (RSI and ISUAL) and also part of AOCS (Attitude and Orbit Control Subsystem), namely the star sensors, gyroscopes and an IRU (Inertial Reference Unit). The lower deck carries the four reaction wheels and the autonomous propulsion module. The pointing accuracy is < 0.7 km (0.12º); the position knowledge is < 70 m (0.02º).

The fixed solar array uses GaAs cells and consists of two deployable flaps. The entire S/C architecture is designed in such a way as to provide a low roll inertia, a key factor for satellite agility and instrument line-of-sight stability. The S/C provides a body-pointing capability of ±45º in roll and pitch (45º pitch in 60 s, 10º roll in 25 s, 30º roll in 60 s, respectively). The spacecraft wet mass is about 760 kg including 81 kg of propellant (N2H4) mass. The design life is five years or better. 2)3)4)5)6)

• OMU (Onboard Management Unit), developed by Acer Inc. OMU is the satellite's central control unit and the core of the secondary system for central commands and data processing. The objective of OMU is to control the satellite's operations, including the processing and execution of uploaded commands and the collection, processing, storage, and downloading the data of satellite's status.7)

Figure 6: Illustration of OMU (image credit: NSPO)

• Onboard flight software module, developed by Tatung System Technologies Corp. The objective is to manage and control the satellite orbit, position, and operation. The flight software is responsible for the calculation of satellite position/orbit, operation of scientific instrument payload, and communication with ground antenna.

• Sun sensor, developed by Shihlin Electric & Engineering Corporation. The objective is to determine the spacecraft's relative position to the sun according to the direction of the source of sunbeam.

Figure 7: Illustration of the sun sensor (image credit: NSPO)

• S-band antennas, designed and manufactured by Victory Industrial Corp. These antennas are responsible for receiving commands and transmitting microwave signals. They allow the uploading and downloading of the ground commands and satellite information.

Figure 8: View of the S-band antennas (image credit: NSPO)

• Flight harness, designed and manufactured by Aerospace Industrial Development Corp. The harness wirings are used for connecting satellite components and payload and for transmitting power, commands, and signals.

• Vertical installation dolly, designed and manufactured by Taiwan Aerospace Corp. It is the main loading equipment for the satellite integration and testing procedures. It has anti-shock and anti-tilting functions and can prevent people from approaching too close to the dolly.

• Satellite primary structure, designed, analyzed and tested by NSPO and manufactured by Aerospace Industrial Development Corp. and Taiwan Aerospace Corp. The satellite primary structure carries the different satellite components and payloads.

Launch: A launch of ROCSat-2 took place on May 20, 2004 (UTC) on a Taurus-XL vehicle of OSC (Orbital Sciences Corporation) from VAFB, CA (maiden flight of Taurus-XL configuration which offers greater lift capability compared to previous versions of the Taurus rocket).

Note: Following the early-orbit checkout, the initial satellite orbit has been raised from 728 km to 891 km altitude in the period May 23 to June 2, 2004. A total of 32 burns were performed by the propulsion module with 4 burns for inclination change, to enter into the mission orbit with an altitude of 891 km and an inclination of 99.14º.

• August 22, 2016: According to a Taipei Times news report, the FormoSat-2 satellite has been decommissioned after 12 years in operation. The satellite had remained operational eight years past its expected lifespan of five years, but a series of equipment failures finally forced the NSPO (National Space Organization) to pull the plug. NSPO made its last contact with FormoSat-2 during a decommissioning ceremony on August 19 at its headquarters in Hsinchu, bringing an end to the nation's most successful surveillance satellite program. 8)

• July 19, 2016: Over the past 12 years, the FormoSat-2 satellite has acquired 2.55 million photos and is best known for documenting images of Taiwan before and after Typhoon Morakot hit the island, which proved helpful to the government's land valuation efforts. Now, according to Taiwan's China Post online infosite, experts announced plans to decommission FormoSat-2. The satellite will be temporarily replaced with Japan's satellite services until the new FormoSat-5 is launched sometime before the end of 2016. 9)

- Vice President Jough-tai Wang of NARLabs (National Applied Research Laboratories) confirmed this news to local media, saying that though FormoSat-2 was originally designed for a five-year mission in May 2004, the satellite has operated for more than 12 years. However, the satellite has now been deemed unrepairable—one of FormoSat-2's four reaction wheels malfunctioned in 2011 and the spacecraft has been running on the three remaining reaction wheels—then another one failed last month.

- Before the new satellite FormoSat-5 can be put into service, NARLabs said the plans are to rely on platforms established by international organizations, including Japan's Sentinel Asia and UNOSAT, to retrieve images. Wang said that the government is still mulling over how to handle the retired FormoSat-2, whether to let the satellite become space junk or return to Earth.

• October 2015: With over 11 years of on-orbit operations (> twice its design life), the FormoSat-2 mission has not only fulfilled the civilian needs on land utilization, agricultural and forest planning, disaster assessments, and environmental monitoring, but its imaging data has also been distributed to international users for specific applications. The unique feature of a daily revisit capability (due to the spacecraft's agility) has been demonstrated to be significant and very useful for post disaster assessment and environmental monitoring. During its lifetime, FormoSat-2 has acquired imagery which covers ~8 times the land area of the Earth. 10)

- In 2010, FormoSat-2 joined the Sentinel Asia Constellation Initiative as an action to align with its commitments and frameworks in the endeavors to build and strengthen a disaster resilient community in the Asia-Pacific region. The Sentinel Asia Initiative has been operation since 2006 as an international cooperation platform under the APRSAF (Asia-Pacific Regional Space Agency Forum).

- Under the fully engaged operations scenario and with a mission life of 5 years, it is inevitable that the satellite encountered many anomalies, either permanent or temporary. In average, about 30 anomalies and 4.5 AROs (Automatic Reconfiguration Orders) occurred every year. In particular, one attitude gyro (totally four in cold redundancy) and one reaction wheel (totally four in hot redundancy) have been failed. A dark line appears in the images taken by ISUAL. Also, 6 orbital maneuvers have been performed to bring FormoSat-2 from its drifted trajectories back to the original deployed trajectory. Basically, FormoSat-2 still keeps about 70 % of the designed functions and capabilities after 11 years. But further failure of gyro or reaction wheel might drastically reduce its capability. 11)

- One of the reaction wheels failed in 2011. The satellite attitude maneuver was then performed with the other three reaction wheels. As a consequence, the agility of the satellite is reduced to ~50%. The primary telecommand interface of another reaction wheel was found anomalous in 2014, so that the satellite is now operated using the redundant reaction wheel. The project is facing a failure if only two reaction wheels remain operational. Hence, an investigation was started to figure out how to control the satellite to stay in a safe (stabilized) mode. Simulations of the attitude control using two reaction wheels show that the attitude can be stabilized to the Sun pointing within 60º to satisfy the safe mode power consumption. — In 2015, FormoSat-2 continues to respond quickly to emergency event imaging for the world. Figure 11 shows the image of Tianjin explosion which occurred on August 12, 2015. 12)

Legend to Figure 11: A series of explosions that killed over one hundred people and injured hundreds of others occurred at a container storage station at the Port of Tianjin on 12 August 2015. The first two explosions occurred within 30 seconds of each other at the facility, which is located in the Binhai New Area of Tianjin, China. Fires caused by the initial explosions continued to burn uncontrolled throughout the weekend, repeatedly causing secondary explosions, with eight additional explosions occurring on 15 August.

• The FormoSat-2 spacecraft and its payload are operating nominally in 2015. 13)

• October 2014: After more than 10 years on orbit, FormoSat-2 is providing continued service with its two payloads, one scientific - ISUAL (Imager of Sprites and Upper Atmospheric Lightning), and one Earth observation - RSI (Remote Sensing Instrument), operate in eclipsed and sunlight times alternatively. Hence, the onboard as well as the ground resources are being used effectively. 14)

WHIS is the largest ice shelf in the Arctic with a size of about 400 km2. It is located at 74.75° longitude west and 83.13° latitude north on the northeast coast of Ellesmere Island, Nunavut, Canada. - WIS has a size of about 150 km x 110 km and is located on the southwest side of the Antarctic Peninsula.

- In support of the IPY (International Polar Year) activities, FormoSat-2 imagery was acquired from the spring of 2006 to the spring of 2008. Among them were 307 targets in northern polar areas, including glaciers and ice shelves in Canada, Greenland, and Russia, as well as 37 targets in Antarctic areas, including the Wilkins Ice Shelf, the Crane Glacier, and the Mertz Glacier. FormoSat-2 provides high-resolution imagery (2m in Pan, 4 m MS) in terms of daily, weekly, monthly, seasonally, and yearly changes in the targets.

From an orbital altitude of 891 km, FormoSat-2 is able to observe the polar regions with a side-looking capability of ±45º (Figure 12). Figure 13 illustrates imaging areas of polar regions of FormoSat-2 indicated by pink-colored strips between 2006 and 2009.

- Current state of health of FormoSat-2: Due to its complexity in mission operations, the acceleration/deceleration frequencies of reaction wheels for attitude maneuver and the charge/discharge cycles of the battery for power supply are much more heavily than other similar satellites. These are the major reasons to cause deterioration of the relevant components. Under such fully engaged operational scenario for 10 years, it is inevitable for FormoSat-2 to encounter many anomalies, either permanent or temporary. In particular, one gyro (out of four) and one reaction wheel (out of four) were failed permanently. Also, from time to time a sequence cyclic overload on flight software may trigger a temporary the AROs (Automatic Reconfiguration Orders).
Since FormoSat-2 still keeps about 80% of its initial design functionality, more monitoring images on the Polar Regions can be expected in the future (Ref. 14).

- In the field of TLE observations, [Ref. 14) IAC-14-B4.4.1] ISUAL had recorded more than 35,000 events in 10 years with 73.93% elves, 6.54% red sprites, 5.81% halos, 13.42% blue jets and 0.30% gigantic jets. After the launch of FormoSat-2 on 20 May 2004 and the completion of instrument check on-orbit, ISUAL caught its first image on 4 July 2004. Tables 2 and 3 show the annual observation time and number of TLEs observed by ISUAL from 4 July 2004 to 31 May 2014, respectively. The global distribution of the ISUAL observation events are shown in Figure 14.

With a design life of 5 years, the FS2 has been working 10 years in orbit. Enormous observation results have been obtained by its payloads, the ISUAL and RSI. Also, significant research results have been reached through the study of the observations. Further investigations include the correlations between TLEs and associated sferics, the energy spectra of the parent lightning of ISUAL observed elves, the secondary sprites, the correlations between ISUAL TLEs and sunspots as well as solar cycle, the possible sudden stratosphere warming signature, etc. are underway.

Year

Aurora (hrs)

TLE (hrs)

Airglow (hrs)

Total (hrs)

2004

1.01

406.09

6.77

413.86

2005

1.12

1,050.95

5.45

1,057.52

2006

9.06

1,274.17

93.38

1,376.61

2007

11.99

1,424.16

136.13

1,572.28

2008

17.78

1,317.78

188.61

1,524.17

2009

5.76

1,402.64

175.09

1,583.49

2010

0.0

1,408.46

85.47

1,493.93

2011

0.0

1,454.55

0.0

1,454.55

2012

3.25

1,221.99

153.19

1,378.44

2013

0.0

1,329.73

66.20

1,386.93

2014 (up to May 31)

0.0

662.80

0.0

661.80

Total (%)

49.97 (0.36)

12,943.32 (93.09)

910.29 (6.55)

13,903.58 (100)

Table 2: Accumulated observation hours of ISUAL from 4 July 2004 to 31 May 2014

Year

Elves

Sprites

Halos

Blue Jets

Gigantic Jets

Total

2004 (%)

588 (77.37)

68 (8.95)

67 (8.82)

37 (4.87)

0 (0.00)

760 (100)

2005 (%)

1,420 (73.46)

217 (11.23)

165 (8.54)

124 (6.41)

7 (0.36)

1,933 (100)

2006 (%)

2,212 (75.88)

201 (6.90)

258 (8.85)

239 (8.20)

5 (0.17)

2,915 (100)

2007 (%)

2,653 (73.92)

255 (7.11)

277 (7.72)

398 (11.09)

6 (0.17)

3,589 (100)

2008 (%)

2,643 (75.15)

188 (5.35)

163 (4.63)

507 (14.42)

16 (0.46)

3,517 (100)

2009(%)

3,046 (75.19)

261 (6.44)

218 (5.38)

510 (12.59)

16 (0.40)

4,051 (100)

2010 (%)

3,195 (72.75)

250 (5.69)

234 (5.33)

697 (15.87)

16 (0.36)

4,392 (100)

2011 (%)

3,394 (72.46)

300 (6.40)

258 (5.51)

718 (15.33)

14 (0.30)

4,684 (100)

2012 (%)

3,016 (73.98)

217 (5.32)

190 (4.66)

718 (15.33)

15 (0.37)

4,077 (100)

2013 (%)

2,795 (73.23)

244 (6.39)

162 (4.24)

612 (16.03)

4 (0.11)

3,817 (100)

2014 (%) to 5.31

1,036 (72.45)

99 (6.92)

51 (3.57)

239 (16.71)

5 (0.35)

1,430 (100)

Total (%)

25,998 (73.93)

2,300 (6.54)

2,043 (5.81)

4,720 (13.42)

104 (0.30)

35,165 (100)

Table 3: Annual numbers of TLEs observed by ISUAL from 4 July 2004 to 31 May 2014

Figure 14: Global distribution of 35,165 TLEs observed by ISUAL from 4 July 2004 to 31 May 2014 (see legend at left upper corner: green for elves, red for sprites, orange for halos, blue for blue jets, and black for gigantic jets), image credit: China University of Science and Technology, Taiwan and NSPO

• 2014: The FormoSat-2 mission is operating nominally. On May 21, 2014, FormoSat-2 will have been on-orbit for 10 years, with continuous monitoring for Taiwan and the world (the design life of 5 years has been doubled). 15)

- The remaining on-board propellant is 27 kg, which is sufficient for orbit maintenance of another 10 years.

- FormoSat-2 is currently in a good state of health, as investigations of the trending data of each subsystem verified (including RSI, C&DH, TT&C, Power, Thermal, Propulsion, and AOCS). The spacecraft experienced malfunctions of one gyro and one reaction wheel.

• Fall 2013: The FormoSat-2 satellite has been operated on orbit for more than 9 years (design life of 5 years). The RSI instrument is operated in the daytime (sunlit period), while the ISUAL is active during the nighttime (eclipse period) of the orbit. 16)

To meet both mission objectives and to accomplish conflict-free planning and scheduling, the mission operations have been more complicated than traditional dedicated Earth remote sensing satellites. For instance, the usage of the satellite's limited onboard resources, especially the SSR (Solid State Recorder) and the power status, needs to be fully cross-checked before a command load generation for either mission objective of payload.

- Malfunction of the IRU (Inertial Reference Unit) and noise of gyro A: The IRU contains 4 gyros, 2 processing units and 2 power supplies; its configuration is illustrated in Figure 15. Gyro input axes are arranged in a skewed redundant configuration such that any of three of four gyro axes provides a full set of three-axis data, i.e., the gyros works in a 3 among 4 cold redundancies for the inertial attitude estimation purpose. Two processing units and two power supplies (PSs) are also redundant, i.e., only one for each is used in operation. In summary, the IRU features four gyros in a tetrahedron configuration, with a full redundancy for power supply and electronics.

The IRU anomaly was first found on February 7, 2006, the time was not yet the completion of 2 years on orbit operation. The configuration of using PS2/CPU2 and gyro A/B/C has been operated smoothly for about seven months. Unfortunately, the 2nd IRU malfunction happened in September 2006. This anomaly led to the 2nd IRU reconfiguration with gyro A off. From that time, the IRU was configured to use PS2/CPU2 and gyro B/C/D. Thanks to the full redundant considerations and cross-strap design, the IRU could be used without degrading its functions and performances even undergone these two malfunctions.

- Malfunction of the RW (Reaction Wheel) interface (Figure 16): The design concept of the reaction wheels in FormoSat-2 is to use a cluster of four wheels in hot redundancy, in order to take advantage of the degree of freedom to avoid wheel rate zero-crossing. The RWA (Reaction Wheel Assembly) consists of one RW and a single channel WDE (Wheel Drive Electronics).

The RW malfunction occurred in November 2009, while FormoSat-2 has been operated for five and a half years, exceeding design life around 6 months. The project managed to find a solution for this problem.

- For more than 9 years of fully engaged operations, the FormoSat-2 spacecraft has encountered many anomalies, including 43 AROs (Automatic Reconfiguration Orders) and lots of error events. Most of the anomalies were transient incidents and the satellite could be switched back to normal situation shortly without interrupting the routine operations. The only permanent failures were the IRU malfunction and gyro-A noise and interface malfunction between RW3 and TIF (Telecommand Interface) board. However, since the failure of IRU/gyro was replaced with spares, the handicapped three-wheel configuration with the degraded maneuver capability did cause significant impacts on the mission operations.

Except for the two permanent malfunctions and three-wheel induced operations impacts, the FormoSat-2 spacecraft behaved quite well. It is expected to be operated on orbit for the next decade under such an experienced mission operations team (Ref. 16).

• The FormoSat-2 spacecraft and its payload are operating nominally in the summer of 2012 - in its 9th year on orbit - providing imagery for the domestic needs of Taiwan as well as for the international community. Astrium GEO-Information Services is the sole distributor of the Formosat-2 satellite's imagery outside of Taiwan.
The daily revisit capability of FormoSat-2 is due to its high agility (FOR of ±45º), its high resolution imagery and quick response; they are a great asset for the coverage of disaster events on a global scale. This service has been requested quite frequently by countries stricken by disasters. - The spacecraft is in good health and expected to make more contributions in its extended mission life. 17)

• On March 11, 2011, an earthquake of magnitude 9.0 struck Japan, and the resulting 10 m high tsunami swept the country's northeast coast within 15 minutes. The United Nations Platform for Space-based Information for Disaster Management and Emergency Response started the International Charter on Space and Major Disasters to coordinate the satellites of different countries in acquiring images of disaster affected areas.18)19)

Figure 17: Quick look images of three strips of Sendai, Fukushima, and Ichihara on March 13, 2011 (image credit: NSPO)

Figure 18: FormoSat-2 images of the Watari District, Mayagi Prefecture on March 11, 2011 (prior to the disaster, left) and on March 12, 2011 after the disaster on the right (image credit: NSPO)

FormoSat-2 responded rapidly to Japan's needs of information about the disaster-stricken regions. Once the first image of the affected area after the disaster was acquired and downloaded at noon on March 12, it was processed within 2 hours and published on GEMDAS (Global Environment Monitoring and Disaster Assessment System).

The role that GEMDAS played in the Japanese disaster can be attributed to three factors. The daily-revisit orbit and high agility of FormoSat-2, the automatic image processing system of the satellite (F2 AIPS), and the web-based geospatial information system used to make the images public all contributed to the effective disaster response (Ref. 18).

• The FormoSat-2 spacecraft and its payload are operating nominally in 2011. Although the mission is beyond its design life of 5 years, the health of the spacecraft is such that NSPO expects continued observations until the follow-on mission is launched.

Note: The FormoSat-2 mission is to be followed by the FormoSat-5 mission of NSPO with a launch scheduled for 2014. The PDR (Preliminary Design Review) of FormoSat-5 was completed on Dec. 16, 2010. 20)

• The spacecraft and its payload are operating nominally in the fall of 2010. The satellite successfully complements existing high spatial resolution imaging satellites such as SPOT-5, IKONOS, and QuickBird, among others, with its unique capability of daily revisits worldwide. FormoSat-2 has now been operated beyond its mission lifetime of five years and is in its extended mission phase. 22)23)24)

- Besides providing imagery for the domestic needs of Taiwan, the spacecraft is frequently being used to deliver high-resolution imagery for event monitoring applications. For the recent large disasters in the world, like southern Asia tsunami, Sichuan earthquake, typhoon Morakot over Taiwan, and Chile earthquake, FormoSat-2 took the first images almost every time, and provided the intensive monitoring images to domestic and international organizations for aftermath relief.

- Example of spacecraft agility: When the Sichuan earthquake occurred on May 12, 2008, FormoSat-2 was able to take imagery over the disaster area during its first pass; this was followed immediately by a +30º roll maneuver to dump the data to the NSPO ground station nearly 3000 km away in real-time. The agility of the spacecraft is in particular ideal for disaster monitoring support by shortening the data transfer time significantly.

- In the first five years of operation, ISUAL (Imager of Sprites and Upper Atmospheric Lightning) recorded more than 12,000 TLE events. Among them, ELVES (Emissions of Light and Very Low Frequency Perturbations from Electromagnetic Pulse Sources) are found to be the most abundant type (~80%) of TLEs (transient luminous events), whereas sprites and halos only combine to account for ~20%. The distribution of sprites and halos closely resemble that of the cloud-to-ground lightning. However, nearly 60% of ELVEs occurred over the ocean, a feature indicating that the high peak current lightning is more abundant over the ocean (Ref. 22).

• After 5 years of operations, the spacecraft is still in a generally very good state-of-health condition and is expected to provide its services for several years (3 to 5) to come. 25)

• In Feb. 2009, NSPO issued a RFP (Request for Proposal) for the FormoSat-5 Remote Sensing Instrument (RSI) to provide continuity of observation data. FormoSat-5 is planned for launch in 2013. 26)

• The spacecraft and its payload are operating nominally as of 2009. So far, the FormoSat-2 imagery has also been used in many applications on a global scale, such as disaster monitoring, land-use (change) detection and environmental monitoring.

• When the Sichuan Province of southwest China was hit by an earthquake of magnitude 7.9 on May 12, 2008, FormoSat-2 was among the first providing its imagery of the stricken region to the Chinese Academy of Sciences. 27)

• In the timeframe 2007/8, the high-resolution imagery of FormoSat-2 was being used by the government of Brazil to plan field interventions to stop illegal deforestation in progress in the Amazon basin. 28)

• In 2006, the IRU power supply failed and the behavior of gyro-A turned out to be rather noisy. Due to these events, the project was forced to use the redundant devices. Thanks to the full redundant design of the spacecraft operations could be continued (Ref. 22).

• In May 2006, NSPO implemented a F2T (FormoSat-2 Terminal) in Svalbard, Norway, to extend the data acquisition capability of the mission.

SOH (State-of-Health) trending during two years of mission operations (May 2004-May 2006) on five major subsystems, including AOCS and EPS (Electric Propulsion Subsystem), and one major payload have been conducted. All parameters were well within specifications. The C&DH and the TT&C do not exhibit any SOH problems so far. 29)

• In April 2005, NSPO implemented a terminal in Kiruna, Sweden, to serve as an additional receiving station.

• The high-resolution FormoSat-2 imagery was used extensively to monitor the devastation on the coasts of south-east Asia caused by the Tsunami that struck the Indian Ocean region on Dec. 26, 2004 (the epicenter of the undersea earthquake was off the coast of Sumatra, Indonesia - killing more than 225,000 people in 11 countries; in retrospect the earthquake was referred to as the "Great Sumatra-Andaman earthquake"). The imagery of FormoSat-2 proved extremely helpful to disaster relief activities and area reform tasks.
Following the Tsunami monitoring activities, imagery from FormoSat-2 has supported Hurricane Katrina relief in the Gulf of Mexico (August 2005), the October 2005 Pakistani earthquake recovery, and other natural disasters.

• Mission operations started in June 2004 (the checkout and performance verification for satellite bus and RSI were started on May 21 and completed through June 2004; all performance requirements of FormoSat-2 had been successfully verified in orbit). Besides providing imagery for the domestic needs of Taiwan, the S/C is frequently being used to deliver high-resolution imagery for event monitoring (coverage of the Tsunami in Asia on Dec. 26, 2004 and thereafter, coverage of Hurricane Katrina in Aug. 2005, coverage of Typhoons on the Pacific, coverage of earthquake regions, etc.). The spacecraft is operating nominally as of 2006. 30)31)32)33)

• NSPO has contracted to SPOT Image S. A. for the international distribution of FormoSat-2 images since June 2004.

• On 4 July 2004, ISUAL successfully observed the first images of sprites, sprite halo, and elves.

• It took 3 days to check the performance and function of all subsystems of satellite bus, and 11 days to raise the satellite from 723 km parking orbit to 888 km mission orbit. Then the primary payload, a remote sensing instrument (RSI), was turned on and the first image was taken on 4 June 2004 (Ref. 33).

Sensor complement: (RSI, ISUAL)

Generally, the RSI is operated during the daytime (requiring sun illumination) while the ISUAL instrument is active during the nighttime phase of the orbit. Without any payload activities during an orbit, the spacecraft is sun-pointing by default to maximize its energy. The spacecraft attitude during the eclipse phase is geocentric (nadir pointing) for RSI thermal regulation and for communication coverage purposes. The mode transition from sunlit to eclipse orbit takes place automatically. For RSI observations, the attitude pointing is geocentric by default.

RSI (Remote Sensing Instrument):

RSI is a pushbroom-type imager built by EADS Astrium SAS, France. RSI is made up of the camera and IPU (Instrument Processing Unit). The camera itself consists of the optical subassembly, the secondary structure, and the FPA (Focal Plane Assembly). The Korsch telescope (mirrors & structure) design and the focal plane structure are being made of SiC (Silicon Carbide). The video electronics, sequencer, DC/DC converter, and compression cards comprise the IPU. 34)35)36)37)

RSI utilizes the agility of the satellite bus for stereo imaging over a specific region, continuous imaging over a slender region, and mosaic imaging over a large region. The imaging capability is 8 minutes per orbit, and the imaging areas during one cycle can be one 3000 km x 24 km continuous strip, two 100 km x 24 km stereo pairs, four 100 km x 24 km strips, or eight scenes.

Figure 22: Schematic structure of RSI (image credit: NSPO)

In-flight radiometric calibration: Although the RSI instrument was calibrated on-ground prior to launch with an uncertainty of 5%, the in-flight calibration has to be done regularly with the pre-flight calibration data as reference. The in-flight calibration includes two major parts: absolute and relative radiometric calibrations. Use of test sites in the ground segment. NSPO and CNES are using the following test sites: La Crau test site (43.56ºN, 4.86ºE) and desert sites Libya 1 (24.42ºN, 13.35ºE), Libya 2 (25.05ºN, 20.48ºE), Libya 3 (23.15ºN, 23.1ºE) and Mauritania 2 (20.85ºN, 8.78ºW), respectively. These sites are selected because of their spatial uniformity and temporal stability of the surface reflectance. 38)39)

ISUAL (Imager of Sprites and Upper Atmospheric Lightning):

ISUAL is a joint international research program of NSPO, UCB (University of California at Berkeley), National Cheng Kung University of Taiwan, and Tohoku University, Japan. The objective is to observe the natural upward lightning discharge phenomena toward the ionosphere on top of the troposphere, referred to as TLEs (Transient Luminous Events). The requirements call for: 40)41)42)

• To determine the location and timing of luminous phenomena above thunder clouds to investigate their spatial, temporal and spectral properties

The instrument consists of four elements: a) intensified sprite imager, b) a six-channel spectrophotometer, c) a two-channel array photometer, and d) an electronics package. The imager is a staring-type frame CCD camera, taking 180 frames/s with a resolution of 512 x 80 pixels in a FOV of 20º x 3.15º. In Figure 24, the large disk of the ISUAL instrument contains six color filters, which can be rotated to select wavelength for measuring the emission spectrum of red sprites. ISUAL is operated in three modes:

• The sprite continuous mode to take images with a sample rate of 100 Hz

• The sprite burst mode with a sample rate of 1000 Hz

• The auroral mode at a constant rate of 1 sample/s.

All the instruments are mounted on a common platform and boresighted in the same direction. The imager is equipped with a six-filter wheel. ISUAL monitoring occurs during each nightside pass. The source data are stored and compressed in a separate 128 MByte memory. The data are downlinked in S-band. The total data volume is about 1 Gbit/day.

The two array photometers are identical in every aspect, except one fitted with a blue band filter and another equipped with a red band filter. The wavelength selections for the two array photometers are 360-450 nm and 525-880 nm, respectively.

Figure 24: Illustration of the ISUAL instrument with sprite imager (left) and spectrophotometer at right (image credit: NSPO)

The imager is designed to capture five images in quick sequence. The imager operates continuously and five data frames are captured when the photometer signals the presence of a flash event in the field of view. This method of operation obtains high temporal resolution framing of the image.

• During the period July 4, 2004 to May 31, 2013, ISUAL recorded 31,369 TLEs (Transient Luminous Events); of this total sum, there were the following classes of TLEs according to their appearances: 73.65% elves, 6.53% red sprites, 6.00% halos, 13.51% blue jets, and 0.30% gigantic jets. 43)

Due to the orbital characteristics of FORMOSAT-2, unobserved gaps exist between each two orbits. Also, the ISUAL has its detection threshold. An estimated global occurrence rate is 24 TLEs per minute, but this number is still much less than the possible potential rate of 195.

Year

Elves

Sprites

Halos

Blue Jets

Gigantic Jets

Total

2004

588

68

67

37

0

760

2005

1420

217

165

124

7

1933

2006

2212

201

258

239

5

2915

2007

2653

255

277

398

6

3589

2008

2643

188

163

507

16

3517

2009

3046

261

218

510

16

4051

2010

3195

250

234

697

16

4392

2011

3394

300

258

718

14

4684

2012

3016

217

190

639

15

4077

2013

937

91

53

370

0

1451

Total (%)

23,104 (73.65)

2,048 (6.53)

1,883 (6.0)

4,239 (13.51)

95 (0.30)

31,369 (100)

Table 5: Number of TLEs observed by ISUAL from 4 July 2004 to 31 May 2013

Year

Aurora

TLE

Airglow

Total

Orbits

Hours

Orbits

Hours

Orbits

Hours

Orbits

Hours

2004

21

1.01

1225

406.09

19

6.77

1265

413.86

2005

24

1.12

3064

1050.95

34

5.45

3122

1057.52

2006

159

9.06

3821

1274.17

305

93.38

4285

1376.61

2007

217

11.99

4128

1424.16

405

136.13

4750

1572.28

2008

217

17.78

3823

1317.78

572

188.61

4612

1524.17

2009

102

5.76

4192

1402.64

272

175.09

4566

1583.49

2010

0

0.00

4054

1408.46

255

85.47

4309

1493.93

2011

0

0.00

4140

1454.55

0

0.00

4140

1354.55

2012

41

3.25

3479

1221.99

613

153.19

4133

1378.44

2013 (May 31)

0

0.00

1340

484.89

314

66.20

1654

551.09

Total

781

49.97

33,266

11445.69

2789

910.29

36836

12405.95

Table 6: Accumulated observation orbits and hours of ISUAL from 4 July 2004 to 31 May 2013

Figure 30: The first Sprite image captured by ISUAL on July 4, 2004 over the Philippines (image credit: NSPO, Ref. 31)

Ground segment:

The basic elements of the ROCSat-2 ground segment are the SOCC (Spacecraft Operations and Control Center) and the XAS (X-band Acquisition System) located in Hsinchu, Taiwan. SOCC in turn consists of MOC (Mission Operations Center), MCC (Mission Control Center), SCC (Science Control Center), FDF (Flight Dynamics Facility), and GCN (Ground Communications Network). FormoSat-2 X-band imagery reception is also made available to third parties (international partners) with their own ground stations through cooperative agreements. 44)45)

The information compiled and edited in this article was provided byHerbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).